Fuel cells convert hydrogen and oxygen to water, releasing energy as usable electricity without employing combustion as an intermediate step. Unfortunately, the use of fuel cells has been limited especially where a rapid change in electricity demand is required such as in residential applications. The problem is that the rate of hydrogen supply to the fuel cell must rapidly change in order to accommodate varying electrical loads.
One could use a reservoir of hydrogen from which to supply the fuel cell, and the replenishment of hydrogen to the reservoir would therefore not be subjected to accommodating the rapid changes in hydrogen demand. However, such a solution is impractical, especially for residential use, due to difficulties and risks associated with such storage. Moreover, hydrogen storage equipment adds to the size and cost thereby reducing the attractiveness of a fuel cell for residential use. Alternatively, electricity could be stored in batteries, which serve as a buffer between the fuel cell system and the electrical load. Batteries, especially of the volume required to meet the needs of residential units, also add to the cost and size of the fuel cell system. Moreover, batteries have a limited life and must be replaced. Another approach is to store electricity in a super capacitor. While the size and cost of a super capacitor may be attractive, the disadvantage is the limited storage capacity.
Ideally, hydrogen would be generated on-site on an as needed basis for the fuel cell by the reforming (e.g., steam reforming and autothermal reforming) of fuels such as methanol, ethanol, natural gas, propane, butane, gasoline and diesel. Such fuels have high energy storage densities, have conventional storage protocols and have a nationwide supply infrastructure.
Although technology exists for the generation of hydrogen by reforming fuels, the implemented production processes are not able to quickly change the rate of hydrogen generation so as to be useful in a residential fuel cell application. For instance, hydrogen is widely produced for chemical and industrial purposes by converting suitable fuel materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas. Such chemical and industrial production usually takes place in large facilities that operate under steady-state conditions.
On-site hydrogen supply for fuel cells used in smaller mobile and stationary facilities, including residential-scale facilities, poses substantial problems even without the added complexities of operating at varying production rates. For instance, hydrogen generators for fuel cells must be smaller, simpler and less costly than hydrogen plants for the generation of industrial gases. Furthermore, hydrogen generators for use with fuel cells will need to be integrated with the operation of the fuel cell such that energy storage requirements are minimized. Moreover, the hydrogen generators must in combination with the fuel cells, be economically viable both in terms of purchase cost and cost of operation, and they must be sufficiently compact to meet consumer desires.
The challenge associated with providing smaller scale hydrogen generators is readily apparent from the number of unit operations required to convert fuel to hydrogen suitable for use in a fuel cell. The fuel must be brought to temperatures suitable for reforming which are often in excess of 600° C. The fuel is reformed to produce hydrogen and carbon monoxide, and the reformate is subjected to water gas shift at lower temperatures to convert carbon monoxide and water to hydrogen and carbon dioxide. Residual carbon monoxide is removed from the hydrogen-containing gas. Additionally, pre-treatment operations are generally required to treat the fuel to remove sulfur, a catalyst poison.
These unit operations must be conducted in an energy efficient manner. Consequently, the overall process should be highly heat integrated. As can be readily appreciated, changes in hydrogen production would be expected to take some time as each of the unit operations and heat exchange operations respond. The severity of the problem in changing hydrogen generation rates is exacerbated in that the range of operation of residential units needs to be quite wide, often the turndown ratio must be at least 5:1.
The difficulties in providing a hydrogen generator for use with fuel cells is further exacerbated because carbon monoxide is a poison to fuel cells. The water gas shift reaction is the primary operation used in a hydrogen generator to remove carbon monoxide generated by the reforming of the fuel. Any upset in the operation of the water gas shift reactor can result in an increase in carbon monoxide that must be removed in downstream treatment of the hydrogen-containing gas. While redundant capacity for carbon monoxide removal (e.g., a selective oxidation) may be used in downstream operations to handle spikes in carbon monoxide production, such an approach will incur a penalty in process efficiency and product purity, as well as compactness and cost of the system. Accordingly, the hydrogen generator must be able to accommodate changes in the hydrogen production rate without adversely effecting the water gas shift operation.
Another problem area in the providing hydrogen generators, especially compact hydrogen generators for fuel cell systems, is to cool the reformate to temperatures suitable for the water gas shift reaction. The use of indirect heat exchange has posed problems due to the inherent lag time required when the production rate of hydrogen is changed. Proposals have included cooling by injecting liquid water into the reformate. The injection rate can rapidly respond to a change in the hydrogen production rate and adequate cooling can be obtained with relatively small amounts of liquid water due to the high latent heat of vaporization. Additionally, as the water gas shift reaction is an equilibrium reaction, the additional water has some benefit in shifting the equilibrium to the production of hydrogen.
U.S. Pat. Nos. 6,162,267 and 6,375,924 disclose a reformer and a separate vessel containing a high temperature water gas shift zone and a low temperature water gas shift zone for generating hydrogen with reduced carbon monoxide content. Water is introduced as a spray above each of the shift zones to control temperature.
US Patent Application Publication 2002/0152680 discloses a fuel cell system in which liquid water is injected between the reformer and the water gas shift reactor to cool the reformate. The liquid water is atomized and/or injected on a high surface area material to assist in the cooling.
However, the injection of liquid water poses difficulties in that the water must be essentially completely vaporized prior to contact with the water gas shift catalyst. The presence of liquid water on the water gas shift catalyst can result in deterioration in performance, thereby increasing the potential of carbon monoxide breakthrough to the fuel cell. Additionally, atomization of liquid water and the use of high surface area contact surfaces such as steel wool, ceramic pellets, and honeycomb monoliths which serve to prevent the passage of liquid water to the water gas shift catalyst, pose disadvantages. For instance, atomization nozzles may not be able to perform adequately over the wide range of hydrogen production rates sought for residential units, and atomization nozzles may require maintenance. The use of high surface area structures results in a pressure drop, may not be effective in assuring complete vaporization of the water, and additional energy will have to be consumed in compression of the gases passing through the hydrogen generator.
An unpublished effort known to the inventor used a coiled tube within the passage between the reformer and the water gas shift reactor. Liquid water was introduced into the tube. Indirect heat exchange with the reformate converted the liquid water to steam, and the steam was released at the end of the coil. Difficulties existed in obtaining a uniform temperature reduction across the cross section of the passage and in providing a nozzle at the end of the tube for introducing the steam into the reformate.
US Patent Application Publication 2002/0094310 discloses a compact fuel processor using a plurality of modules stacked end-to-end.
US Patent Application Publication 2001/0002248 discloses a hydrogen generating apparatus having heat integration and the asserted ability to provide a constant hydrogen concentration over a range of hydrogen production rates.
US Patent Application Publication 2001/0014300 discloses a reformer controlling apparatus using downstream detectors to control the fuel to air ratio and the amount of fuel and air.
Apparatus and processes are sought which can handle a wide range of throughputs and overcome the inherently slow thermal response of system components, especially the reformer and water gas shift reactor with associated heat exchangers, without risking undue fluctuations in carbon monoxide production. Moreover, the technology should be economically viable for a compact unit providing hydrogen to a fuel cell and not render the compact unit so complex that it is not sufficiently reliable for residential use.